Electrical wiring contacts which are used to effect electrical terminations in electrical wiring devices such as electrical plugs, receptacles and connectors, are conventionally fabricated from a single blank of metal having good electrical conductivity. Metals used for this purpose are typically composed of various alloys of brass and copper because they provide the requisite conductivity and the spring rate needed to apply the necessary spring pressure against the mating male contact blade. Contacts formed by bending or otherwise forming a single blank of metal into appropriate contact shapes are often preferred because they require a minimum inventory of piece parts and usually provide excellent reliability in usage.
An important consideration in the design of unitary female contacts is to ensure that the optimum contact pressures established in manufacture is also maintained during usage. It is especially important that the yield point of the metal composition forming the contact not be exceeded. If the yield point of the metal is exceeded, the contact blade will take a permanent set and subsequently may not provide the desired contact pressures.
Prior art female contacts of unitary construction often utilize an elongated, cantilevered contact blade composed of a suitable spring material mounted opposite and parallel to another elongated contact blade. A gap formed between the opposing free ends of the two blades receiving the male contact. The cantilevered blade may be analogized, from a structural standpoint, to a beam which is fixed at one end but free at the other end. Deflection of the beam about its fixed end results from engagement with the male contact blade and produces shear and tensile stresses in the beam. If these stresses exceed the yield point of the material forming the beam, the beam will take a permanent set and lose its effectiveness as a resilient contact. Therefore, unless supplementary reinforcement means of some type is employed to prevent overstressing of contact blades, or very specially alloyed metal compositions are used, the female contacts function reliably for only one range of male contact thickness. When resorting to the use of specially alloyed contact compositions, the contact manufacturers are confronted with substantial quality control problems in assuring that the correct relative amounts of the various compositions are present in the contacts for optimum characteristics.
In the case of electrical wiring devices, various constraints are imposed upon the maximum length permitted of the female contacts and therefore, it is usually not possible to decrease the bending stresses in the contact blades by merely increasing their blade lengths. Additionally, to prevent overheating of the contacts, the male blade thickness typically increases with increases in their nominal current-carrying rating. For example, a 20 ampere locking-type plug has a nominal blade thickness of 0.060 inch (1.5 mm.), whereas a 30 ampere plug of the same type has a nominal blade thickness of 0.070 inch (1.8 mm.). Usually, if the female contact is designed to provide the requisite restraining forces for the 20 ampere blade, it will not provide the requisite restraining forces for a 30 ampere rated plug without running the risk of exceeding the yield point of the female contact. For this reason, different inventories of female contact sizes are usually required to accommodate plug blades of different current ratings and thicknesses. The aforementioned problems are overcome in large measure by the female contact of the instant invention.